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Old 12th August 2006, 11:22 AM   #1
JoergST is offline JoergST  Germany
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Default Problem with Velocity V2100

Hello, I have a problem with an amplifier Velocity V2100.
In this amplifier there are 6 IRFZ 44 for the power supply. 3 of them were destroyed. I have changed all of them with a IRFZ44N.
When I connected the remote to 12V the amplifier need 16 A. I think this is to much. My 12 V powersupply is dancing on my table where the amplifier is connected. So I solder the power diodes out and turn on the amplifier. In this case the amplifier needs 140mA and my powersupply stands still on the table
Then I solder the power diodes and turn on the amplifier.
The multimeter shows me 16A and my powersupply started up to dance on the table.
I looked at the amplifier transistors KTB 688 and KTD718 but they are ok.

So what can I do?

Regards Jörg
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Old 12th August 2006, 03:05 PM   #2
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This is the type of situation where a current limiting resistor is helpful. I use a 2 ohm 25 watt tubular ceramic type but an automotive headlamp like an H6054 (or similar) can also be used. f you have a 2 ohm dummy load (not a speaker), that can also be used as the limiter. The resistor is inserted in the B+ line between the power supply and the amp. The limiter doesn't work in every instance but for most all problems that don't involve a direct short circuit across the B+ and ground terminals, it's very useful.

When using a limiter, you can test various points with little chance of damaging components. Of course, the amp should be mounted in the sink for the best protection against damage.

With a limiter and the amp powered up, I would suggest that you check the voltage across ALL of the emitter resistors. If you find one or more that have significantly higher voltage than the others, that will give you an idea of which channel is drawing excessive current.

If there is no excessive voltage across any emitter resistor, check the rectifiers.

140ma is very low current for an operating power supply. Did you confirm that the supply was actually being driven when the rectifiers were out of the circuit. If you don't have a scope, measure the DC voltage on the gates of the FETs (black meter lead on chassis ground) with the amp on (rectifiers removed). If you read ~5-6 volts DC, the supply is likely being driven. You can also try measuring the AC voltage on the secondary transformer windings but some handheld meters don't have enough bandwidth to give any usable readings.
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Old 12th August 2006, 04:25 PM   #3
JoergST is offline JoergST  Germany
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Default Velocity V 2100

Hello, I have found something. The amplifier is a 2 channel amplifier with 2 KTD718 and 2 KTB 688 per channel.
Infront of the to channel there is a 2SC3198.
On the picture you can see I solder out all transistor from one channel. When the 2SC3198 is solder in the position Q 202 (small red cirkle) you can switch the amplifier on an the current is 0,75A.
When I change the position from the 2SC3198 to Q201(big red cirkle with Transistor) the amplifier has got 16 A. I think something in the platine behind must be damaged.

Regards Jörg
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Old 12th August 2006, 05:09 PM   #4
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That's likely the bias compensating transistor. Removing the transistor from the circuit will drive the output transistors on, in effect turning a class AB amp into a high biased class A amp. This can cause the output transistors to fail within a few seconds if they are not clamped to the heatsink.
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Old 14th August 2006, 07:01 PM   #5
JoergST is offline JoergST  Germany
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Hello, now I will buy a new 2SC3198 and will install him in the amplifier. when I have the transistor, I will tell you what happen.

Regards jörg
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Old 19th August 2006, 07:48 AM   #6
JoergST is offline JoergST  Germany
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Talking Velocity V 2100

Hello I have found the mistake in the amplifier. A small Transistor 2SC3198 was defect. It is between the 2 KTD718 and 2 KTB 688. Now I have installed a new transistor and the amplifier works with a curent from 0,71A by normal noise.

thank you all for your help.

Regards Jörg
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